1. Field of the Invention
The present invention relates to a developing device that uses two-component developing agent, and an image forming apparatus provided with the developing device, and more particularly relates to the agitation mechanism of the two-component developing agent.
2. Description of the Related Art
In image forming apparatus such as photocopiers, printers, facsimile machines, printing machines, and soon, the recorded output is obtained by processing an electrostatic latent image formed on a photosensitive member or latent image carrier using a developing device to obtain a visible image, then the visible image is transferred onto a sheet or the like.
The developing agent used in developing may be a one part developing agent containing magnetic or non-magnetic toner only, or it may be a two-component developing agent that is a mixture of toner and a carrier. Two-component developing agent includes toner and carrier to carry the toner. When the two-component toner is agitated and mixed, the toner becomes charged by the frictional electrostatic effect, so that the toner can be electrostatically attracted to the electrostatic latent image on the photosensitive body.
A developing device frequently includes a developing sleeve that draws up the developing agent onto its peripheral surface by magnetic force and supplies the developing agent to the electrostatic latent image on the photosensitive body, and an agitating sleeve that supplies agitated and mixed developing agent to the developing sleeve. The developing agent from which the toner was consumed in visible image processing of the electrostatic latent image on the photosensitive body is recovered in the developing device, mixed with replenished toner, agitated, and again used for developing. Therefore, the developing agent used in a developing device constituted in this way must maintain a constant toner temperature and electrostatic charge, in order to obtain stable toner images. The toner temperature is adjusted depending on the quantity of toner consumed in developing and the quantity of replenished toner, and the amount of electrostatic charge applied by the frictional electrostatic charge when mixing the carrier and toner. Therefore in this type of developing device, the two-component developing agent formed from the toner and carrier is sufficiently agitated, so that the toner temperature distribution becomes uniform, the toner becomes charged, and the toner images are made stable.
However, the amount of charge varies depending on various factors, so even under uniform agitation conditions the amount of charge will vary. For example, these factors can include the surrounding environment (temperature and humidity), the reduction in the carrier charging performance with time, the variation in amount of charge on the toner with the area of image output, in other words, there is a variation in the amount of charge depending on the holding time of the toner. Also, variation in the amount of charge is caused by the amount of newly replenished toner. This is because the greater the quantity of toner, the greater the time required for mixing and agitation.
Conventionally, normally in this type of developing device, in the short period of time until the replenished toner is scooped up and supplied to the developing sleeve, the toner must be dispersed and charged using an agitation screw. However, if toner consumption (consumption and supply) is high, the replenished toner can be scooped up by the developing sleeve without being sufficiently dispersed, and as a result, contamination of the surface of the photosensitive member due to insufficient charging or contamination of the surrounding parts due to dispersion of toner in the air can occur.
In order to prevent dispersion of toner in the air and image defects caused by this type of insufficient mixing and agitation, in other words insufficient toner charging, providing a plurality of mixing and agitation members within the developing device to improve the mixing and agitation has been proposed. However, although the initial problem is solved in this way, the agitation stress on the developing agent is increased, which causes a reduction in the charging capacity due to degradation of the developing agent with time. As a result, the amount of charging is unstable, so the problems of dispersion of the toner in the air and contamination of surfaces with time remain. When stress is applied to the developing agent, the charging capacity is reduced by the removal of the coating film, the charging capacity is reduced by the embedment of additives in the toner, and the fluidity is changed, and so on. The changes in fluidity is also a cause of reduction in image quality due to changes in charging capacity and changes in the transfer ratio.
In order to solve these problems, a constitution has been proposed in which the agitation unit is provided in a location separate from the developing unit, and the developing unit and the agitation unit are connected by circulation means. Then, after the toner concentration and amount of charging of the developing agent within the agitation unit is made optimum for the developing unit, the developing agent is transported to the developing unit has been proposed. See, for example, Japanese Patent Application Laid-open No. 2008-003560 (hereafter referred to as Prior Art 1), Japanese Patent Application Laid-open No. 2007-193301 (hereafter referred to as Prior Art 2), Japanese Patent No. 3349286 (hereafter referred to as Prior Art 3), Japanese Patent Application Laid-open No. H11-143196 (hereafter referred to as Prior Art 4), and so on.
In Prior Art 1, agitation is carried out by rotating a shaft on which a plurality of agitation members has been provided. This type of agitation method is widely used.
In Prior Art 2, a constitution is disclosed in which the developing agent is circulated between the developing device and an agitation device, and the agitation device is provided with an agitation member such as a paddle or a screw or the like. Developing agent that has been agitated and mixed within the agitation device is transported to the developing device. Then agitation and mixing of the developing agent contained within the developing device is carried out by a plurality of agitation and transport screws provided in the developing device.
In Prior Art 3 a constitution is disclosed in which a screw whose axis is vertical is disposed within the agitation device as described above, and developing agent that is dropping down is made to contact developing agent that is raised by the rotation direction of the screw, and charging and agitation is caused by the friction.
In Prior Art 4, a constitution is disclosed in which an agitation member and a transport screw having their shafts in the horizontal direction are provided. Toner that drops from a toner replenishment tank is frictionally charged by the agitation, then the developing agent is transported to the developing unit via a transport screw using a Mohno pump.
However, in the toner agitation unit, it is necessary to appropriately charge the toner for replenishing the developing agent whose toner concentration has been reduced after developing, and transport it to the developing device so that the developing agent will have the appropriate toner concentration. However, the developing agent is continuously circulated on a path, and the toner content changes after developing, so it is difficult to always maintain a constant amount of charge and toner concentration without changing the circulation amount (flow rate).
Each of the above prior art documents will now be examined from the above point of view. In the constitution according to Prior Arts 2 and 4, the agitation operation can be easily made uniform, but a long amount of time is required to agitate all the contents of the agitation unit within which the agitation screw is housed. Therefore, to reduce the time it is necessary to make the agitation unit larger, which increases the quantity of developing agent held, which is then held longer in the agitation unit. Moreover, increasing the agitation speed of the agitation member in order to reduce the time causes new problems such as increasing the stress on the developing agent, and increasing the load on the drive system due to the reaction load from the developing agent.
Also, in the constitution disclosed in Prior Art 3, an agitation screw that raises the downward falling developing agent upward is used. However, frictional charging occurs when there is contact between the downward falling developing agent within the range of the developing agent being raised by the agitation screw. Therefore, the charging function is limited to the size of the area occupied by the agitation screw, and it is difficult to expect uniform charging capability of the developing agent in all the areas containing developing agent. In particular, the developing agent that is falling downwards relative to the developing agent that is being raised by the blades of the screw extends throughout all the developing agent areas of the agitation unit. Therefore, to apply sufficient amount of charge to the toner it is necessary to increase the length of the screw in its axial direction. Also, instead of this constitution which results in a larger size, if the rotation speed of the screw is increased, the new problem of damage to the toner will be caused, as discussed for the constitutions disclosed in Prior Arts 2 and 3.
On the other hand, it is possible to consider widening the agitation area by providing another agitation blade on the outer periphery of a paddle or screw having its axis in the vertical direction as disclosed in Prior Art 2. However, in this constitution, there is the problem of discharge of insufficiently charged developing agent in the gap between the members used for agitation or the gap between the agitation blade positioned on the outer periphery and the internal surface of the agitation unit. Therefore increasing the rotation speed of the members used for agitation can be considered, but in this constitution the new problem of acceleration of degradation of the developing agent is caused.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Application Laid-open No. H07-134481, Japanese Patent Application Laid-open No. H10-063081, Japanese Patent Application Laid-open No. H10-240007, and Japanese Patent Application Laid-open No. 2004-085879.